This invention is generally directed to developer compositions, and more specifically, the present invention is directed to improved conductive developer compositions possessing stable electrical properties over extended periods of time. In one embodiment of the present invention, there is provided certain improved developer compositions useful over a wide range of toner concentrations, which compositions have stable triboelectric charging properties, and desirable narrow charge distributions. Further, with the developer compositions of the present invention, there results substantially no undesirable background when these compositions are selected for the development of electrostatic latent images. Moreover, the developer compositions of the present invention desirably retain their conductivity over extended time periods.
Developer compositions for use in electrostatographic imaging systems are well known, these compositions generally being comprised of resin particles, pigment particles, and carrier particles. Many of the prior art developer compositions also contain therein various additives, such as fatty acid salts, Aerosil, and other similar materials. There is for example, described in British Pat. No. 1,442,835 developer compositions containing polystyrene resin particles, pigment particles, at least one polyalkylene compound selected from polyethylene and a polypropylene, and as an optional ingredient a paraffin wax and a metal salt of a fatty acid. According to the disclosure of this patent, the addition of a metal salt of a fatty acid provides for the improved compatibility of the polyalkylene compound in the resin component. Also, apparently the dispersion capability of the paraffin wax, pigment particles, and other toner additives, such as charge enhancing additives, is further improved with the use of zinc stearate. For example, it is stated in Column 4, beginning at line 45 of this patent that the non-sticking property of the resulting toner particles are improved, that the stability of the toner composition involved can be marketedly enhanced, and that the toners life can be sharply prolonged without being subjected to a change in frictional charge characteristics, even when the toner is used for a long period of time. Furthermore, it is indicated in this patent that the resulting toner compositions may also be improved in their moisture resisting properties.
Disclosed in U.S. Pat. No. 3,320,169 is a developer composition comprised of three components, namely magnetic carrier particles, toner particles consisting of a discrete mixture of pigmented resin particles, and an aliphatic acid having from about 10 to 26 carbon atoms, and/or salts of such aliphatic acids. Examples of fatty acids disclosed include saturated or unsaturated acids containing from 10 to 26 carbon atoms such as lauric, stearic, oleic, and the like. Preferred additives are calcium stearate and lithium stearate. Examples of thermoplastic resin particles disclosed in this patent include polystyrene resins, acrylic resins, asphalt, polyvinyl resins, and the like.
There is disclosed in U.S. Pat. No. 3,914,181, developer compositions comprised of finely divided toner particles clinging to the surface of specific carrier particles comprised of nickel zinc ferrite beads, or manganese zinc ferrite beads, which carrier particles apparently have substantially uniform properties including uniform triboelectricity, magnetic permability, and electrical conductivity. These carrier particles are obtained by preparing a slurry of ferrite forming metal oxides in a liquid, spray drying the slurry of metal oxides to form substantially metal oxide beads, followed by sintering the resulting spherical metal oxide beads. It is indicated in this patent that various toner resin materials can be selected for the developer composition involved, including phenolformaldehyde resins, methacrylate resins, polystyrene resins, polypropylene resins, epoxy resins, and the like.
It is also known that many of the developer compositions described in the prior art are useful in magnetic brush development systems, wherein there is provided developed images with excellent solid area development and desirable line development. In magnetic brush development, the electrostatic image contained on the photoreceptor imaging member, creates an electrical field for development, which results when the triboelectric attraction between the toner particles and the carrier particles is less than the electrostatic forces exerted by the latent electrostatic image. With this imaging system, as well as other similar imaging systems, there is generally selected for the developer composition carrier particles containing an insulating polymeric coating. These insulating coatings while desirable for some purposes can limit development since, for example, as the toner particles are attracted to the electrostatic latent image, there remains on the magnetic brush a counter charge which cannot dissipate rapidly in view of the presence of the insulating coating on the carrier particles. Accordingly, the accumulated counter charge effectively causes a reduction in the development field, thereby reducing the tendency for the toner particles to migrate from the magnetic brush to the latent electrostatic image contained on the imaging member. Thus, development ceases prior to neutralization of the latent image. This problem can however be avoided by incorporating multiple development rolls into the electrostatic imaging device, whereby the electrostatic latent image can be developed by successive magnetic brushes. Also, when developer compositions containing insulative carrier particles are selected for developing latent electrostatic images, the distance between the imaging member and the roller containing the developer composition must be maintained at a constant close proximity, since the strength of the development field is an inverse function of the imaging member to developer roll spacing for these particles.
Many of the above problems can be eliminated when there is selected for the developer composition electrically conductive carrier particles, in that for example, with such particles there is provided a leakage path for the counter charge associated with the removal of the toner particles from a magnetic brush, thus preventing the self limiting development phenomenon present with insulative developers. Moreover, as the conductive developer composition provides a current path from the development roller into the bristles of a development brush, the development field originating from the imaging member terminates at a point in close proximity to the tips of the magnetic brush. Therefore, the effective spacing of the development field for conductive developer compositions is not completely dependent on the distance between the imaging member and the development roll, thus a high development field can be achieved with wider spacings, approaching, for example, from about 1,500 microns to about 5,000 microns. As a result it is believed that conductive developer compositions provide a higher level of image development and a lower level of background development than insulative developer compositions. Thus the substitution of conductive developer compositions for insulative developer compositions in electrostatographic systems can allow for (1) a reduction in the number of development rolls, (2) reduce the tolerance required for the imaging member development roll spacing, and (3) provide for a development system where image development is enhanced, and undesirable background development is minimized.
Nevertheless, conductive developers do not function entirely in the manner desired, since they possess unstable conductivities, thus such developers generally increase in conductivity, or decrease in conductivity, with usage. For example, conductive developer compositions containing insulating toner particles and carrier particles consisting of a steel core coated with an oxide in an appropriate amount so as to provide the desired level of conductivity, can after extended use become more conductive in view of the loss of the oxide coating. This results in developer failure, causing a loss of the development of fine lines in a direction perpendicular to the development process, a discharge of the latent electrostatic image, and short circuits from the developer roll to the imaging member substrate. Similarly, conductive developer compositions can become insulative with use, particularly those conductive developer compositions comprised of highly impactable toner particles, that is toner particles that will permanently adhere to the surface of the carrier particles. This adherance provides an insulative region on the carrier particles, and increases the number of toner particles adhering to the carrier surface, causing the carrier particles to become less conductive, resulting in decreased developability. In imaging devices containing an automatic density control system, a decline in developability as a result of reduced conductivity in the developer composition can result in complete development failure as the addition of toner particles to the developer will limit the contacts between the carrier particles causing, for example, a decrease in the output density and resulting in some instances in images with high background densities, as well as excessive undesirable machine contamaination.
Furthermore, the conductivity of a developer composition is dependent on and related to the properties of the insulative toner particles contained therein. Thus, the conductivity of a developer composition is a function of the number of toner particles contained on the surface of the conductive carrier particles. Also the conductivity for various developer compositions containing carrier particles and toner particles can be related to the carrier conductivity in that experimentally the logarithm of the developer conductivity is a linearly decreasing function of the toner concentration. For example, if the ratio of the carrier conductivity to developer conductivity is defined as R, the conductivity sensitivity parameter is defined as the natural logarithm of R divided by the toner weight percent concentration. The logarithm of the developer conductivity at a toner concentration TC, will be the logarithm of the carrier conductivity minus the product of the conductivity sensitivity parameter and the toner concentration. Similarly, the developer breakdown potential can be related to the carrier breakdown potential in that experimentally the developer breakdown potential is a linearly increasing function of the toner concentration. Thus, if the slope of this relationship is defined as the breakdown sensitivity parameter, (obtained by dividing the difference between the developer and carrier breakdown potentials by the toner concentration), the developer breakdown potential at a toner concentration TC is obtained from the carrier breakdown potential plus the product of the breakdown sensitivity parameter and the toner concentration. While it is appreciated that these relationships generally correspond to experimental data over various ranges of toner concentrations generally selected for use in xerographic imaging devices, such relationships may not generally accurately describe data obtained at very high toner concentrations.
There thus continues to be a need for improved developer compositions, particularly improved conductive developer compositions. Additionally, there continues to be a need for improved developer compositions which retain their conductivity for extended time periods. Moreover, there continues to be a need for improved developer compositions containing stable triboelectric charging values, and narrow charge distributions, in order to allow for the use of such compositions over a wide range of toner concentrations. Further there continues to be a need for conductive developer compositions which when selected for use in xerographic imaging systems results in images with minimum or substantially no undesirable background development.